The progressive attack on polished glass surfaces caused by exposure to high humidity conditions was studied with photomicrographs and by measuring the visible–light transmissions. Carefully polished specimens of five commercial glasses representing a wide range of chemical durability were exposed to relative humidities of 95, 73, and 57% at 90°C. for a total of 1296 hours. Photomicrographs taken at frequent intervals show the degree of surface alteration as a function of time and humidity and furnish some basis for characterizing the course of weathering. For some glasses, the transmission losses did not correlate simply with the amount of visible attack; gains in transmission were noted frequently as the attack progressed. One glass in particular exhibited a general trapsmission gain of 1% after 24 hours of weathering and maintained this gain throughout the tests. Surface films produced on the least resistant glasses caused greater transmission loss in the blue region of the visible spectrum than in the red. Following a dilute‐acid wash, transmissions approached, or in some cases exceeded, values characteristic of the original polished condi